A surface aerator generally comprises a power motor mounted on a float or a rigid structure of a purification plant. The motor drives a shaft provided with impeller means such as a paddle wheel or a propeller generally combined with a static diffusion or deflection head. There are several kinds of aerators among which one can make a distinction between low speed aerators (about 60 rev/min) and high speed aerators (about 700-1800 rev/min).
A low speed aerator mainly comprises an impeller shaped as a conical paddle wheel having a large diameter, a part of which extends below the surface level of the water and which is driven by an electric motor through a speed-reducing gear. The efficiency of such aerators is very good in clean water.
High speed aerators are provided with a propeller shaft directly connected to the motor. The water is forced upwardly at high speed by a marine-type propeller and is diffused at a considerable angle above horizontal by a static deflector or by a trailing edge. According to their application, these aerators have more or less good efficiency in water loaded with sludge.
In such prior art devices, an impeller such as a simple marine-type propeller has been rotated beneath the surface of the liquid to cause the liquid to rise through a surrounding, downwardly extending volute. The upward flow has then been directed against a rotating or stationary diffusion head which has deflected the liquid into a flow path extending at a considerable angle above horizontal. As a result of this type of prior art deflection, the velocity of the liquid leaving the diffuser head has actually been less than its upward velocity through the volute, leading to a loss in kinetic energy and a short flight path for the liquid after leaving the diffuser. The liquid has tended to break into a rather coarse spray, also. As a result, the rate of oxygen transfer to the liquid has been less than desired and the energy consumption by the pump motor has been too high for a given degree of purification.
In other prior art devices, a further paddle-wheel deflector has been mounted above the propeller and on the same shaft, but above the water line. In this case, the stream of water leaving the volute encounters the paddle wheel blades, which also deflect the water away as a rather course spray into a flow path extending at a considerable angle above horizontal. The liquid hitting the blades on the deflection paddle wheel has undergone a sort of hydraulic shock as it has been forced abruptly to change directions, again leading to a loss of kinetic energy and a short flight path for the liquid after leaving the deflection paddle wheel.
Other prior art attempts to improve the performance of surface aerators have involved the use of various types of stationary and spinning nozzles which have opened toward the periphery of the aerator and through which the upwardly moving liquid has been deflected to depart from the aerator as a fast moving stream, rather than a spray, into a flow path extending at a considerable angle above horizontal. Because the liquid has left the nozzle as a stream rather than a spray, the rate of oxygen transfer has been less than desired.
Concerning the efficiency of such prior art apparatuses, care must be taken to distinguish between the efficiency of the aerator as a pumping device, which might be stated in terms of volume of liquid pumped per kilowatt hour; and the efficiency of the aerator as an aerator, which might be stated in terms of volume of oxygen added to the liquid per kilowatt hour. While an aerator may be found to have good efficiency as a pump, it does not follow at all that it will have good efficiency as an aerator.